Metal doesn’t generaly cause issues in the microwave unless there are sharp corners.
The whole inside of the microwave is metal (*you will notice curves and round corners everywhere, as well as surfaces made to specific angles).
It does reflect the waves, however, which can mess with the heating distribution.
Put a metal spoon in the coffee to heat it up more evenly. Sounds weird, metal in the microwave, but it doesn’t spark when it’s in liquid.
Or you could just stir it afterwards with that spoon.
Metal doesn’t generaly cause issues in the microwave unless there are sharp corners.
The whole inside of the microwave is metal (*you will notice curves and round corners everywhere, as well as surfaces made to specific angles).
It does reflect the waves, however, which can mess with the heating distribution.